1. Field of the Invention
The present invention relates to a circuit board and a display device. More particularly, the present invention relates to a full monolithic circuit board and a display device including such a circuit board.
2. Description of the Related Art
The market of a flat panel display is now being expanded with highly-advanced informatization. Flat panel displays such as a non-self-emitting liquid crystal display (LCD), a self-emitting plasma display (PDP), an inorganic electroluminescent (inorganic EL) display, and an organic electroluminescent (organic EL) display, are known, and such displays are being actively researched and developed.
According to conventional display devices, a driving circuit and the like is mounted outside a display panel, and a thin film transistor (TFT) that is formed in the panel is typically used for pixel switching. In recent years, a display device including a monolithic circuit board, where a driving circuit and the like is arranged on a substrate inside the panel, is being researched and developed. TFTs included in such a display device need to have higher characteristics than those of TFTs included in the conventional display devices.
In the full monolithic circuit board, TFTs need to have higher performances than those of TFTs for pixel switching, and characteristics need to be hardly varied among the TFTs. So TFTs having such transistor characteristics are being researched and developed. As a typical way of improving performances of the TFTs, the gate wiring is designed to be as thick as possible for reduction in resistance.
A technology of reducing a resistance of a gate wiring in a TFT used for pixel electrode switching by providing the gate wiring with a two-layer structure is disclosed (for example, see Patent Document 1). According to this technology, the TFT, which is used as a switching element for a pixel electrode, includes a base coat film 111, a semiconductor layer 112, a gate insulating film 113, a gate electrode 114, and an interlayer film 116, stacked in this order on a glass substrate 110, as shown in FIG. 10. As shown in FIG. 11, the gate wiring portion is formed to have a two-layer structure composed of a first gate wiring 115 as a lower layer and a second gate wiring 118 as an upper layer, which can reduce the resistance of the gate wiring portion.